1. Field of the Invention
The present invention relates in general to a communication system with medium access control (referred to hereinafter as MAC) sub-layers provided respectively in a pluraty of mobile terminals and a base station, and more particularly to a communication system suitable for the implementation of a multimedia communication service.
2. Description of the Prior Art
Recently, communication systems have very rapidly been developed in every country. Digital communication systems proposed up to now can be classified into a frequency division multiple access (FDMA) type, a time division multiple access (TDMA) type and a code division multiple access (CDMA) type according to how accesses are established between a base station and mobile terminals. Only one of the above-mentioned various access systems may selectively be used in some countries or regions. Alternatively, different access systems may simultaneously be used in other countries or regions. For example, the CDMA communication system and a global system for mobile communication (GSM) are simultaneously used in Moscow. To this end, each mobile terminal or base station (or network) is required to comprise two types of modulation/demodulation modules to compatibly control the two types of access systems. This requirement runs counter to the international communication equipment trend of lightness, thinness, smallness and compactness and becomes a primary factor of weakening the competitiveness of communication products.
On the other hand, even if a single communication system is employed in a certain region, products may be different in function according to their options and operating systems of mobile terminal manufacturers, other communication equipment manufacturers and communication service operators.
In this connection, a study is being made of increasing the compatiblty of communication equipment to provide a communication service to users using the same communication system in the same region, regardless of manufacturers and communication service operators. The representative example may be a Digital European Cordless Telecommunication (referred to hereinafter as DECT) system. This DECT system has been developed under the necessity of cordless communication capable of accepting data communication requirements to a certain degree while giving the first consideration to voice communication. Accordingly, the DECT system can establish interoperabibty among communication equipment of different manufacturers to provide the users with various communication services regarding voice and data as extended optional services, as well as basic services.
FIG. 1 shows a protocol stack used in a conventional DECT system. As shown in this drawing, the conventional DECT system is composed of a physical layer (PHY) 10, a MAC sub-layer 20, data link control layers (DLC1 and DLC2) 30 and 40 and a network layer (NWK) 50.
The PHY 10 of the DECT system functions to divide a given radio spectrum into physical channels. These divisions are made at two domains, or time and frequency. For the frequency and time divisions, a TDMA operation is performed with respect to multiple radio carriers. For example, in the conventional DECT system, ten carriers are provided at a frequency band between 1880 MHZ and 1900 MHZ.
The MAC sub-layer 20 of the DECT system basically takes charge of two main functions. The first function is to select a physical channel and set or release connection of a call on the selected physical channel. The second function is to multiplex control information together with upper layer information and error control information into a packet in the form of a slot or demultiplex the packet into such information.
The DLC1 30 and DLC2 40 act to provide reliable data links to the NWK 50. Also, the DLC1 30 and DLC2 40 are closely connected with the MAC sub-layer 20 to provide the upper layer with data integration higher than that provided single-handed by the MAC sub-layer 20. In FIG. 1, a C-plane is common to all application layers and provides very reliable transmission links for internal control signals and a limited amount of user information traffic. A U-plane is adapted to provide a variety of alternative services, which are optimized for specific requirements.
The NWK 50 of the DECT system is a main signal layer of the protocol stack. In the conventional DECT system, the NWK 50 employs a configuration similar to that in an ISDN layer 3 protocol (ETS 300 102) and provides functions similar to those in the ISDN layer 3 protocol.
In the above-mentioned conventional DECT system, the MAC sub-layer activates or deactivates a pair of physical channels to control bearer creation, maintenance and release operations, an empty physical channel selection operation and a received signal quality estimation operation.
However, the conventional MAC sub-layer has been developed to be suitable for voice-centered communication systems. For this reason, the conventional MAC sub-layer may be used to perform radio data communication to a certain degree, but it will cause many functional problems when being applied to multimedia communication service-based next generation communication systems. Noticeably, the next generation communication system will require highly compatible communication equipment to entirely solve various communication service problems resulting from option differentiation between communication equipment manufacturers and communication service operators and various problems resulting from different communication manners.
Further, the next generation communication system must be able to select many options because it will use multimedia information such as voice, video and text and require very excellent speech quality.
However, the MAC sub-layer of the conventional DECT system is adapted to provide only two functions, the former selecting a physical channel and the latter multiplexing control information into a packet in the form of a slot or demultiplexing the packet into such information. For this reason, the conventional MAC sub-layer is not applicable to the above-mentioned multimedia communication service.